---------------------------------------------------------- RotaxEngines-List Digest Archive --- Total Messages Posted Sun 03/09/08: 2 ---------------------------------------------------------- Today's Message Index: ---------------------- 1. 03:23 PM - Re: Re: 912 with low RPM--- new vibration problem (Les Goldner) 2. 05:35 PM - Re: 912 with low RPM--- new vibration problem (Roger Lee) ________________________________ Message 1 _____________________________________ Time: 03:23:39 PM PST US From: "Les Goldner" Subject: RE: RotaxEngines-List: Re: 912 with low RPM--- new vibration problem Thank you for your advice Roger, I followed it and lowered the pitch on my 912ULS.... but now I have a big problem. Although I very carefully changed the pitch of the 3-blade warp drive prop about 3/4ths of a degree, and the maximum RPM is now within the range you suggested, the plane now vibrated a lot more than it did at all but idle setting. Yes I double checked the uniformity of prop pitch and made certain that the blades were seating right.. 3 separate times. So I think the problem is not be related to resetting the prop. I also re-sync'd the carbs, changed the plugs, and cleaned the gas filters; which did nothing to reduce this vibration. There is no difference in vibration using either mag. FYI, the engine was sitting around for over 2-years, sealed in its original box, before I bought it and added oil to the cylinders. It ran fine for 20-hours before it the vibrating started yesterday. The engine oil analysis that was made when the oil was changed at 10-hours, as recommended by Rotax, showed no signs of excess wear. If you or anyone else has any ideas about what could be wrong, I sure would like the information. Thanks, Les > -----Original Message----- > From: owner-rotaxengines-list-server@matronics.com > [mailto:owner-rotaxengines-list-server@matronics.com] On > Behalf Of Roger Lee > Sent: Wednesday, March 05, 2008 2:02 PM > To: rotaxengines-list@matronics.com > Subject: RotaxEngines-List: Re: 912 with low RPM > > --> > > Hi Les, > > Let's take a look at the reasons for the correct rpm at > different speeds. > First the 912 80hp only has 8.5:1 compression where as the > 912 100hp has 10.5:1 compression. The low idle rpm will cause > the 912uls (100hp) to lug or a better term is to chug because > it is fighting against the high compression. The 80 hp > doesn't have that problem because of the lower compression. > Rotax wants the idle on the 100 hp set to at least the 1800 > mark give or take a few rpm. 1400-600 is too low and just > beats on the gearbox and components. Warm up in the morning > should be done at 2000-2400 rpm because it is easier on the > engine and it will not hurt it because the engine is not > under an in flight load. Sitting there warming up at 2400 rpm > has a lot less load on the engine than in flight rpm of 5000. > This came right from Rotax. I just finished their new version > of their service center school. > This engine was designed to run cooler and smoother at higher > rpms. Mid range rpms of 4400-4800 run hotter egt than running > it at 5000-5200rpm and cause more vibration. It can have a > continuous running at 5500, but it will be easier on the > engine's wear over the 1500hr TBO time to run it down around > the 4900-5200 mark. Anything that runs higher and higher rpms > have to burn more fuel, cause more over all stress, friction > and wearing of parts. That's just the way it is when metal > parts run together. There are some point issue with engines > running to low and rpm and of course too high. Rotax designed > this engine to be run for the everyday normal use from > 4900-5200. Yes you can run it all the time at 5500 without > any real issues, but you will encounter more wear and tear on > parts over the life of the engine. As far as running it above > 5500 rpm you can, but for 5 minutes or less. If you do not > have a need for the extra climb performance or you do not fly > at high altitude (7k feet! > or more) then setting the prop to turn 5500 rpm WOT is a > good place to be. If you fly your plane and you are heavily > loaded a good part of the time, fly out of a high density > altitude most of the time or fly high then you may want to > set up the prop to turn 5600-5750rpm WOT. At the higher rpms > you can only fly there a short time anyway like takeoffs, but > if you fly at 8k-10k feet you will be able to turn the prop > faster, make a little more HP at those altitudes and go a > little faster at those high altitudes. This is not guess > work, but I have proved this to many. Having the prop set to > achieve 5500 or a tad more can benefit you on a bad landing > too. If you bounce or flare back into the air and need power > and rpm now to save your butt then a prop set to get 5500 or > more will spin up faster than a prop set for 5100 WOT and > will give you the little extra boost to save your plane when > you are hanging on the edge of a stall. > The everyday torque curve for for the 912uls states close to > this. As the engine rpm climbs so does the HP. The torque > will climb a little faster than the HP. The torque's best rpm > setting is between 4800-5200 rpm then torque starts to drop > off, but HP still climbs as the rpm climbs. No one gets a > true 100 hp out of their engine for several reasons, but > having an engine that only gets 5100-5200 WOT is really doing > the flight performance and the engine a disservice. Having it > set up to run 5500 WOT straight and level is good and if need > be only for specific reasons then more rpm and a little more > hp can be achieved. This engine was et up to be more flight > specific than most know. > > But it is a great engine and almost bullet proof. Treat it > right and it will last for ever. I have some friends that > have 2800-3200 hrs on their 912uls engines and have never had > an issue or rebuild. > > -------- > Roger Lee > Tucson, Az. > > > > > Read this topic online here: > > http://forums.matronics.com/viewtopic.php?p=167885#167885 > > > > > > > > Photoshare, and much much more: > > ________________________________ Message 2 _____________________________________ Time: 05:35:19 PM PST US Subject: RotaxEngines-List: Re: 912 with low RPM--- new vibration problem From: "Roger Lee" Les, Take a really good look at your engine mounts. Don't be gentle. Try and move them with the engine and see if they show any signs of separation or cracking. Has the rpm ever been over 5800? The other big item is you should really get a static and dynamic balance. This is where I would put my money right now. This could easily be the problem. I might be willing to bet money on this one. A static and dynamic balance's should be done by everyone. More people have this problem and don't even know it. If it is out of balance a little you may not feel it in the stick or frame. If it's out quite a bit then you really feel it. This issue is hard on gearboxes engine mounts and all the related components and this is the hardest thing to get people to do. If they can't see it it must be ok or not broken. Wrong! -------- Roger Lee Tucson, Az. 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